In the production of a fluorinated polymer such as a polytetrafluoroethylene (hereinafter referred to as PTFE), a melt-processable fluororesin or a fluoroelastomer by aqueous emulsion polymerization, it is common to employ a fluorinated emulsifier as an emulsifier to prevent an inhibition of the polymerization reaction by chain transfer in the aqueous medium.
A powder of a fluorinated polymer obtainable by coagulation and drying of an aqueous emulsion of the fluorinated polymer obtained by the aqueous emulsion polymerization, is used for various applications. Further, an aqueous dispersion of the fluorinated polymer obtainable by concentrating or applying stabilization treatment, as the case requires, to the aqueous emulsion of the fluorinated polymer, is used for various coating applications or impregnation applications together with various additives.
However, the fluorinated emulsifier to be used as an emulsifier is expensive, and the amount of its use is substantially influential over the production cost of the fluorinated polymer. Further, the fluorinated emulsifier is a substance which is not easily decomposable in nature, and accordingly, in recent years, it is desired to reduce the amount of not only a fluorinated emulsifier contained in a waste water from coagulation of an aqueous emulsion of the fluorinated polymer, discharged from a plant, but also a fluorinated emulsifier contained in an aqueous dispersion of the fluorinated polymer to be commercially sold.
As a method to reduce a fluorinated emulsifier in the aqueous dispersion of the fluorinated polymer, a method has been proposed wherein the aqueous dispersion of the fluorinated polymer itself is contacted with a basic anion-exchange resin. Further, some methods have also been proposed to recover the fluorinated emulsifier from the basic ion-exchange resin.
For example, as a method for eluting an adsorbed, fluorinated emulsifying-acid from a basic anion-exchange resin, a method of employing a mixture of a diluted mineral acid and an organic solvent, specifically a mixed liquid of concentrated hydrochloric acid and an alcohol is disclosed (Patent Document 1). Further, an elution method is also proposed wherein a combination of an aqueous alkaline solution and an organic solvent, specifically a combination of sodium hydroxide or ammonia, and methanol is employed (Patent Document 2). Further, a method is also proposed wherein a basic anion-exchange resin is contacted with an aqueous solution containing a fluorinated surfactant (also called as a fluorinated emulsifier) having hydrochloric acid added thereto, to have the fluorinated emulsifier adsorbed on the basic anion-exchange resin, and then the fluorinated emulsifier is eluted with an alkaline solution containing an organic solvent, specifically a mixture of sodium hydroxide and an alcohol (Patent Document 3). Further, an elution method is further proposed wherein a mixture of ammonia and a water-miscible organic solvent, specifically a mixture of ammonia and methanol, is employed (Patent Document 4).
Patent Document 1: JP-B-63-2656
Patent Document 2: JP-A-2001-62313
Patent Document 3: JP-A-2002-59160
Patent Document 4: JP-A-2003-512931